Earnings per Share/Capital Stock: (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Basic
Net income from continuing operations	$ 18,664,000	$ 15,617,000	$ 43,857,000	$ 35,300,000
Net income (loss) from discontinued operations		(18,000)		3,850,000
Net Income	18,664,000	15,599,000	43,857,000	39,150,000
Less: Distributed earnings to common shareholders	6,766,000	5,236,000	17,316,000	15,311,000
Less: Distributed earnings to participating securities	52,000	38,000	119,000	102,000
Undistributed earnings	11,846,000	10,325,000	26,422,000	23,737,000
Undistributed earnings allocated to common shareholders	11,755,000	10,250,000	26,242,000	23,580,000
Undistributed earnings allocated to participating securities	91,000	75,000	180,000	157,000
Total income available to common shareholders, basic	18,521,000	15,486,000	43,558,000	38,891,000
Weighted average Common Shares outstanding, basic	19,059,000	18,701,000	18,924,000	18,672,000
Basic earnings per Common Share:
Income from continuing operations (in dollars per share)	$ 0.97	$ 0.83	$ 2.30	$ 1.88
Income (loss) from discontinued operations (in dollars per share)				$ 0.20
Net Income (in dollars per share)	$ 0.97	$ 0.83	$ 2.30	$ 2.08
Stock option and restricted stock option outstanding
Options outstanding (in shares)	226,208	668,360	226,208	668,360
Diluted
Common shareholders earnings, basic	18,521,000	15,486,000	43,558,000	38,891,000
Undistributed earnings for dilutive stock options		75,000	180,000	157,000
Total common shareholders earnings, diluted	18,521,000	15,561,000	43,738,000	39,048,000
Weighted average Common Shares outstanding, basic	19,059,000	18,701,000	18,924,000	18,672,000
Stock-based compensation (in shares)	44,000	151,000	114,000	144,000
Weighted average common shares outstanding, diluted	19,103,000	18,852,000	19,038,000	18,816,000
Diluted earnings per Common Share:
Income from continuing operations (in dollars per share)	$ 0.97	$ 0.83	$ 2.30	$ 1.88
Income (loss) from discontinued operations (in dollars per share)				$ 0.20
Net Income (in dollars per share)	$ 0.97	$ 0.83	$ 2.30	$ 2.08
Additional disclosure
Stock options included in the calculation of diluted EPS (in shares)			182,932	425,839
Stock options not included in the calculation of diluted EPS (in shares)	43,276	241,921	43,276	241,921
Anti-dilutive stock options not included in the computation of diluted EPS (in shares)				600
Common Shares issued under Common Share Purchase and Dividend Reinvestment Plan, the 401(k) Plan, and the stock incentive plans			423,590	100,148
Value of Common Shares issued under Common Share Purchase and Dividend Reinvestment Plan, the 401(k) Plan, and the stock incentive plans			12,434,000	2,350,000
Common Shares purchased on the open market under 401(k) Plan and the Common Share Purchase and Dividend Reinvestment Plan			571,652	319,470
Quarterly dividends paid	$ 6,700,000	$ 5,200,000	$ 17,307,000	$ 15,306,000
Quarterly dividends paid, per share of common stock (in dollars per share)	$ 0.355	$ 0.28	$ 0.915	$ 0.82
Restricted Stock Units Member
Share based compensation arrangement
Restricted stock units outstanding (in shares)	148,176	136,703	148,176	136,703